New theoretical research out of McMaster University suggests that Charles Darwin might have been right when he mused that life on Earth started in a warm little pond, and that the window on these perfect conditions has since closed.
To better understand how life started billions of years ago, lead authors Ben Pearce and Ralph Pudritz and their collaborators at Max Planck Institute set out to combine data from many fields, including astrophysics, geology, chemistry, biology and more.
Published in PNAS, the result was a mathematical model of early Earth and its complex chemistry, with a clearer picture than ever of how the first building blocks of life may have formed.
Before the first cells, complex biomolecules needed to be made. Nucleobases, the building blocks for genetic information, likely came from carbon-rich meteorites that hit the Earth. We still find nucleobases on modern day meteorites, and based on the moon’s craters, meteorite strikes were likely more frequent on the young Earth.
Under the right conditions, those nucleobases could have linked up to form long chains of RNA, which are still found in all organisms, can encode genetic information, and can spontaneously self-replicate by folding over and drawing more nucleobases from the environment. RNA is the simplest molecule that fits all the basic requirements for life.
The widely-held competing theory that RNA formed at the ocean floor, close to heated vents in the Earth’s crust, has one major flaw: those locations were always under water. By contrast, ponds on land would have cycled through wet and dry periods, and wet-dry cycles are needed for nucleobases to polymerize into long chains.
Using previously published data to estimate the number of ponds on the early Earth, the rate of meteorite strikes into them, and their potential nucleobase concentration, the team calculated that thousands of ponds would have had conditions favourable for life.
At the same time, there were several factors working against RNA formation. As continents were rising out of the ocean and allowing ponds to form, meteorite bombardment rates were also going down. And while wet-dry cycles are necessary for RNA chains to form, wet ponds lose nucleobases to chemical reactions, and also as water seeps out of them and into the ground. Meanwhile, dry ponds expose nucleobases and their chains to destructive UV light from the sun. This means that RNA polymerization needed to happen quickly, over just a few years, in just one or a few wet-dry cycles.
The authors estimate that RNA polymers would have likely had to form 4.17 billion years ago or more.
This comes hot on the heels of a recent discovery of the oldest known evidence of life on Earth in 3.95-billion-year-old rocks from Northern Labrador, which suggests that by that time, life was already sufficiently abundant to create these carbon-rich sediments.
The authors plan to test their theory at the bench soon, recreating and observing the early Earth’s environment. They hope this will fill in gaps in the chemistry needed to go from nucleobases to RNA strands.
As the search continues for life off the Earth, these insights about early life on Earth may continue to provide new clues.